LPV vs LNAV on 530W

[Jerry 5TJ]

I flew the "C" this morning, IFR in VMC to KSCK and flew the RNAV (GPS) 29R approach. The 530W loaded/activated the procedure and LPV annunciation came at the FAF.   

ASIDE:  The localizer and glideslope information were stable all the way down to touchdown on the centerline at the desired touchdown point; WAAS-augmented GPS can be darn impressive.  

Then I flew to o69 and flew the full RNAV (GPS) 29 approach starting from JIVLI.   Only LNAV came up; LPV is listed on the chart but the 530W did not switch to that mode.    

Anyone know what's happening? 

 

 

[M016576]

If the 430/530 doesn't pass it's accuracy check for the WAAS approach, it won't go into LPV mode. You can't manually switch it "on" or "off": the 430/530W will automatically tell you the "best" approach the GPS is capable of making based on what it's seeing.... It's rare to see it say anything other than LPV, but that doesn't mean that your always guaranteed to be able to shoot the approach down to LPV minimums, just for the very reason you mentioned: sometimes it won't "activate"

[Piloto]

One of the reason for not switching to LPV is due to poor GPS reception. On the GPS Status page verify that some satellite signal levels reach the top of the scale. If none reach the top you may have an antenna or cable problem. This causes not to receive satellites on the horizon that provide accurate lateral navigation.

 

José

[kmyfm20s]

When the LPV doesn't activate and should just hit the procedure button and activate "vector to final" and it will engage the LPV mode. What your talking about happen all the time at my home field all the time. I believe it has to do with the initial approach leg transition to the final approach coarse and the plane not tracking with in a 2 degree limits, at least something along those lines. It happens usually when I'm using the autopilot.

[Marauder]

As was just pointed out, where you are relative to the final approach course can also impact it. I think the 650 manual says that you need to intercept at least 2 miles out from the FAF for it to engage. In your situation where it worked at one location and not another, I would verify that this what is happening by flying it again and giving it a little bit more time.

[Jerry 5TJ]

Thanks for the suggestions.   Here's some more information:

The issue was not with tracking the final approach course for a certain distance, or starting the approach inside the IAP which "Vectors to Final" would indeed fix.

I did check the amplitude of the GPS birds; they do indeed reach the top line on the status page so their RSSI appears to be fine both at zenith and near the horizon.  That was a good suggestion, thanks Jose.

In the event, per the avionics guy today, the problem was....a known glitch with earlier 530W firmware where the nav would NOT initiate WAAS approaches to three-letter airport designators.  So, all was fine at KSCK and KSTS, but a few miles away at O69, no luck with multiple attempts over two days; it just would not go beyond LNAV approach accuracy level.    Now that the firmware version is current the 530W is happy to sequence to LPV approaches at 3 letter airports.

 

[Piloto]

Jerry, what firmware version you had before? I have 3.1 on mine. But I never attempted an instrument approach on a three letters airport. Most of the ones that I fly to have four letters KFXE, TJBQ, CYYR etc. Usually three letters designation (no numbers) is associated with a NAVAID and five letters with an intersection. 

 

Thanks for your observation.

 

José

[Jerry 5TJ]

I did not see the FW version before the tech inserted the loader and updated it.   I heard him say the new version is at level 5.